Cummins Engine Co. of Columbus, Ind., has developed a new engine
retarder that uses silicon nitride ceramic surfaces to resist wear and
increase durability. The retarder, called C-Brake, fits on the N-14
Cummins 14-liter diesel engine, which is a 500-horsepower in-line
six-cylinder power plant used mainly in trucks.

The retarder is a cast-iron block bolted to the engine head to slow
the engine and take some work load off the wheel brakes. As a result,
less maintenance is performed on brake liners.

The ceramic part is an insert produced by Enceratec Inc., a joint
venture of Cummins and Toshiba Corp. of Tokyo. The insert is installed
between an adjusting screw and the brake's master piston. Because
of its wear resistance, the part adds to the retarder's life span,
allowing the system to operate for more than a million miles. Cummins
had previously relied on a hard steel piston surface to combat wear and
boost the retarder's life span to 50,000 miles. Still, the piston
was often the first component to show extreme wear.

During an internal combustion engine's expansion cycle,
cylinder exhaust valves normally remain closed in order for the piston
to receive optimum power from combustion. C-Brake makes use of a
hydraulic circuit to open cylinder exhaust valves during the
engine's expansion stroke. With the retarder in place, the fuel
supply to the cylinder is stopped. Air entering the cylinder exits
through the exhaust, and there is no combustion to send the piston
downward.

Truck drivers often use this feature to slow their vehicles when
they are barreling downhill, said Stephen Smith, chief engineer of the
C-Brake's design. To activate the system, drivers must have their
feet off the accelerator and clutch because the driveline must be
engaged and the cylinder must not receive fuel. Drivers flip a switch in
the dashboard that activates a solenoid in the retarder. This allows oil
into the hydraulic circuit from the engine block.

The entrance of oil serves two primary purposes. First, the oil
pushes a slave piston downward so that the device engages with a
crosshead used to push down on two exhaust valves. Second, the oil
pushes down on the brake's master piston so that the piston
contacts the head of an adjusting screw. The screw is used to set a gap
width, which allows some slack in the valve drivetrain. It also rides on
a push tube that contacts an injector lobe on the engine camshaft.

With the master and slave pistons in place, the retarder is ready
to push down on the valves. This happens as the engine turns and the
camshaft lobe exerts force on the master piston. Hydraulic power is
transferred to the slave piston, which pushes on the crosshead to
activate valve motion.

COPYRIGHT 1994 American Society of Mechanical Engineers
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